In certain wireless, radio frequency (“RF”) communication networks, e.g., those using a CDMA (code division multiple access) spread-spectrum multiplexing scheme, data and other signals are transmitted from one or more fixed base stations to one or more wireless units across a first frequency bandwidth (e.g., a 1.25 MHz bandwidth centered at a first designated frequency) known as the forward link. Transmissions from the wireless units to the base stations are across a second frequency bandwidth (e.g., a 1.25 MHz bandwidth centered at a second designated frequency) known as the reverse link. Generally, each wireless unit is allocated the entire bandwidth all the time, with the signals from individual wireless units being differentiated from one another using an encoding scheme. The forward and reverse links may each comprise a number of physical or logical traffic channels and signaling/control channels, the former primarily for carrying voice data, and the latter primarily for carrying the control, synchronization, and other signals required for implementing CDMA or other communications. Typically, a wireless network is geographically divided into contiguous cells, each serviced by a base station, and/or into sectors, which are portions of a cell typically serviced by different antennae/receivers supported on a single base station.
Concomitant with improvements in electronics technology, recently implemented standards for CDMA-based communication systems provide for the high-speed transfer of data. 1x-EVDO, for example, is an implementation of the CDMA2000® “3-G”/third generation mobile telecommunications protocol/specification configured for the high-speed wireless transmission of both voice and non-voice data. One significant improvement of 3-G CDMA technologies is enhanced forward link power control, which was adopted in the CDMA2000® standard. With enhanced forward link power control, a wireless unit transmits power control commands to a base station to request an increase (up) or a decrease (down) in forward link traffic channel power. This may be done at a rate of up to 800 Hz, for tracking the dynamics of wireless unit speed, fading, path loss, and the like. However, due to power control bit errors, this tracking is typically less than ideal. Specifically, when a call is in soft handoff, power control bit errors can occur differently amongst different handoff legs. This can result in an unintentional loss of synchronization in the traffic channel transmit power at spatially separated base stations. In effect, bit errors occurring on the reverse link cause the forward link power to unintentionally diverge. Unsynchronized forward link power among handoff legs can degrade 3-G forward link capacity. It may also degrade performance to such an extent that the frame error rate (FER) increases, which can result in dropped calls. To mitigate the problem, a forward link reference transmit power level (“Pref”) is used in addition to the power up/down commands. In the presence of power control bit errors, the transmit power is controlled to converge to the Pref level in a manner as set forth in the UMTS (Universal Mobile Telecommunications System) standard. This facilitates the synchronization of forward link power among handoff legs.
To explain further, the act of transferring support of a wireless unit from one base station to another is called “handoff.” Handoff occurs when a call has to be handed off from one base station to another as the user moves between cells. In a traditional, “hard” handoff, the connection to the current base station is broken, and then the connection to the new base station is made. Since all the base stations in a CDMA-based network use the same frequency bandwidths, however, it is possible to make the connection to the new base station without breaking the connection to the current cell. (Each base station with a connection to the wireless unit is referred to as a “call leg” or “leg.”) This is known as “soft” handoff. Soft handoff requires less power, which reduces interference and increases capacity.
For coherent wireless communications such as used in CDMA, pilot signal-assisted channel estimation schemes may be used. The forward link pilot channel/signal is an un-modulated, direct-sequence spread spectrum signal transmitted by the base stations. Pilot signal-assisted methods allow a wireless unit to acquire the timing of the forward link. They also provide a phase reference for coherent demodulation, as well as a means for signal strength comparisons between base stations for use in call handoff. In fact, one of the primary functions of the soft handoff control module/algorithm (in place on the network for controlling soft handoff) is to maintain an “active set” based on the measured pilot signal strength. In particular, a wireless unit periodically measures and reports pilot signal strengths of nearby base stations to the network. As soon as the measured strength of a pilot signal crosses a certain predefined threshold value, the base station transmitting the pilot signal may be assigned as a member of the wireless unit's active set, that is, a member of the group of base stations assigned to concurrently transmit forward link traffic channel data to the wireless unit. Originally, the active set consists of a single base station. After repeated measurements, however, other base stations are added when the measured strengths of their pilot signals exceed the predefined threshold. In a typical soft handoff algorithm, three parameters are considered: “Tadd” (the threshold for adding a base station to the active set), “Tdrop” (the lower threshold for dropping a base station from the active set), and a drop timer “T_tdrop.” If a non-member pilot signal strength exceeds Tadd, the base station transmitting the signal is added to the active set. The wireless unit continues to measure the pilot signal. If it drops below Tdrop, the wireless unit starts a drop timer. If the strength of the pilot signal rises above Tdrop before the drop timer expires, the timer is disabled (in that instance) and reset. Otherwise, upon expiration of the timer the base station is removed from the active set.
As noted, when a call is in soft handoff, power control bit errors can occur in different handoff legs, which may result in a loss of power synchronization. To elaborate, in forward link power control, a wireless unit transmits power control messages over the reverse link, which are received by one or more base stations. The base stations vary the power of traffic channel signals transmitted to the wireless unit depending on the power control messages. However, because there are different signal paths between each of the base stations and the wireless unit, power control messages transmitted over the reverse link may contain bit errors. Because of these bit errors, a power control message may not be interpreted in a manner as originally intended, possibly resulting in a loss of power synchronization.
In current systems, the forward link reference transmit power level Pref is a fixed value independent of forward link quality. Causing the transmit power to converge to Pref in the presence of power control bit errors may help with forward link power mis-synchronization, as mentioned above. However, in some cases the use of Pref in this manner may result in an inefficient utilization of forward link power. For example, the active set may contain a base station with a weak pilot signal relative to the rest of the active set, e.g., due to an unexpired T_tdrop. (In other words, the drop timer T_tdrop has been started for the weak leg, but the weak leg has not yet been removed from the active set.) The weak leg's transmit power is around the Pref level. When Pref is high, this results in a waste of forward link transmission power.